Processes which result in the mass transfer of one or more components contained in a first liquid medium having a given specific gravity, to a second substantially immiscible liquid medium having a different specific gravity by intimately contacting the liquids together are called liquid-liquid extraction processes. Such extraction processes have been used for many years and may be carried out in a number of ways. Typically, the liquid solution to be treated is initially brought into intimate contact with a suitable substantially immiscible liquid which preferentially extracts one or more components from the solution. Since the liquids have different specific gravities, they are then allowed to separate from each other, for example, by gravity, and are recovered. The components that are extracted by such a process may be liquids, solids or ionic species, for example.
In one application of such a liquid-liquid extraction process, components which are soluble in organic solvents can be removed (extracted) from an aqueous solution by intimately contacting the aqueous with a suitable water immiscible organic extractant, followed by phase separation. Similarly, acidic or basic components of an organic solution can be removed by contacting the organic solution with an alkaline or acidic aqueous solution as appropriate.
In yet another application of a liquid-liquid extraction process, an aqueous solution containing an ionic species can be contacted with a liquid "ion exchange material" which forms all or part of an organic medium that is immiscible with the aqueous phase. Upon such contact, the ionic species combines with the ion exchange material (ion exchanger) to thereby form a compound that is soluble in the organic but insoluble in the aqueous. One example of a liquid ion exchanger is a hydroxy oxime exchanger which is useful to extract copper from acidic or basic aqueous solutions containing copper ions. Typical of such a hydroxy oxime ion exchanger is a product sold under the trademark "LIX 64N" by Henkel Corporation, 1844 West Grant Road, Suite 104, Tucson, Ariz., 85745-1273.
As is mentioned above, in liquid-liquid extraction processes, two immiscible liquids are brought into intimate contact for the purpose of the mass transfer of one or more components from one liquid (or phase) to another liquid followed by physical separation of the two liquids. Any device or combination of devices which accomplishes such mixing and separation one time is called a stage. Since in each such stage, mixing of two immiscible liquids takes place and the resulting dispersion is allowed to settle out to thereby separate the phases, such devices or units are usually called mixer-settlers.
In many instances, a liquid-liquid extraction plant includes a plurality of mixer-settler units arranged in series. For example, such an extraction plant can include one or more mixer-settler units that make up an "extraction section" in which a material is transferred from one liquid medium (the feed) into a second liquid medium (the extractant) as is described above. The plant can also include one or more mixer settler units in series with the "extraction section" to provide a "stripping section". In the mixer-settler units of the stripping section the pregnant extractant (extract), i.e., the extractant containing the material transferred to it, is contacted with an immiscible liquid medium capable of removing the transferred material from the extractant. The regenerated extractant can then be recovered from the stripping section and recycled back to the extraction section. Additionally, the liquid containing the transferred material (the raffinate) can either be discarded or treated by various means to recover the transferred material.
In one example of using a multi-stage extraction plant for removing copper ion from an aqueous phase, there can be four mixer-settler units in series. For example, two mixer-settler units in series with each other can be used as an "extraction section" in which an organic medium, such as a mixture of kerosene and "LIX 64N" (ion exchanger), is brought into intimate contact with an aqueous phase, preferably at a pH greater than about 2 and containing copper ion. In the extraction section, the copper ion is removed from the aqueous phase and transferred to the organic medium by means of the "LIX 64N" ion exchanger. Two mixer-settler units can be arranged in series with the extraction units to provide a "stripping section". In the stripping section a solution of sulfuric acid, preferably at a pH less than about 2, can be brought into intimate contact with the copper containing organic phase to thereby remove the copper from the organic as copper sulfate. The regenerated organic can then be returned to the first mixer-settler unit of the extraction section as fresh organic to start the cycle over again and the copper can be recovered from the sulfuric acid solution, for example, by electrowinning.
There are mixer-settler units of a wide variety of designs presently known in the art. For example, U.S. Pat. No. 3,989,467 discloses a unit which includes a mixing tank where liquids are mixed to form a dispersion which then flows into a settling tank located next to the mixing tank. The liquids flow horizontally across the settling tank and during this horizontal flow, three horizontally extending layers develop; the upper and lower layers are formed by the coalesced phases as the liquids separate from the dispersion, while the middle layer, which is usually only a few inches in depth, is a dispersion of one of the liquids in the other. The separated phases are removed from the end of the settling tank remote from the mixing tank by means of suitably located weirs. Attempts to reduce the horizontal cross-sectional area of such settling tanks for a given flow rate of dispersion can result in a disproportionate increase in the depth of the dispersion layer and accordingly, can result in flooding of the settler with dispersion. This results in carryover of one of the liquids in the other and inefficiencies in operation.
The large size of conventional horizontal gravity settlers, such as the unit described above, requires that an undesirably large inventory of organic liquids be maintained in the settler. Additionally, such horizontal units can take up much more ground or floor space than is desired.
U.S. Pat. No. 4,221,658 addresses the problem of undesirably large organic inventories and horizontal cross-sections associated with horizontal units by providing a generally vertical mixer settler unit where both mixing and settling are carried out in the same vessel. The mixing zone, where the immiscible liquids are mixed together to form a dispersion, is at the center of height of the vessel and the coalescing zones into which the immiscible liquids of the dispersion settle out are above and below the mixing zone.
There remains a need in the art, however, for a vertical mixer-settler unit with an improved design for enhancing the mass transfer and extraction as well as the complete separation of the immiscible liquids from the dispersion so that carryover is minimized.